The present invention relates to a screw rotor used as an air compressor or the like.
Referring first to FIG. 1, a conventional screw rotor generally represented by reference numeral 5 comprises male and female rotor elements 3 and 4 rotatable around parallel axes 1 and 2, respectively. The male element 3 is of clover-like cross section and had three ridge portions 6 extending helically in the axial direction while the female rotor element 4 has valley portions 7 extending helically in the axial direction and engaged with the ridge portions 6. The ridge and valley portions 6 and 7 are so designed and constructed that they engage with each other like intermeshing gears to leave a small clearance between each ridge portion 6 and its corresponding valley portion 7.
In using the screw rotor 5 as, for instance, an air compressor, the elements 3 and 4 are rotated by drive means (not shown) in the direction indicated by the arrows to compress and feed the air in the valley portions 7 rearward in the axial direction, as indicated by a white arrow, in a casing (not shown) housing therein the elements 3 and 4.
FIG. 2 illustrates a front view of the screw rotor 5. As described above, small clearance is defined at 8 between the elements 3 and 4. To narrow the clearance will contribute to enhancement of air compression efficiency, but is disadvantageous in that danger of contact between the elements 3 and 4 due to, say, thermal expansion of the same will be increased. In order to overcome this problem, so far the male and female elements 3 and 4 are applied with polytetrafluoroethylene (known as Teflon.RTM.) at 9 in the thickness of tens microns, thereby narrowing the clearnace between the ridge and valley portions. As a result, even when the elements 3 and 4 are caused to contact with each other due to thermal expansion thereof or the like, only the coatings 9 are separated off from the elements 3 and 4. Therefore any accident such as breakdown due to thermal welding between the metallic elements 3 and 4 can be avoided.
Such application of the coatings 9 to the elements 3 and 4 has difficulty in setting the thickness of each coating 9, resulting in tendency of the coatings with a low degree of accuracy. There arises a further problem that the coatings 9, which are made from the same material, tend to weld together upon contact with each other and each coating 9 is substantially separated from the elements 3 and 4, resulting in difficulty in defining an optimum clearance between the ridge and valley portions of the elements 3 and 4.
In view of the above, a primary object of the present invention is to provide a screw rotor which can prevent a pair of rotor elements from contacting with each other and which can operate with leaving an optimum clearance between the elements.
The above and other objects, effects and features of the present invention will become more apparent from the following preferred embodiment of the present invention taken in conjunction with the accompanying drawings.